Various types of hemodialysis apparatus are known. In such apparatus the material interchange between the blood and the dialysate takes place in a dialyzer. The dialyzer has a first flow path for conducting the blood and a second flow path for conducting the dialysis fluid. The flow paths are separated by a semipermeable membrane. The first flow path is part of an extracorporeal blood circulation having a feed line and a return line for the blood, and possibly, a pump promoting the blood flow. The second flow path is connected for the feed and outflow of the dialysate.
The known types of hemodialysis apparatus have substantial differences in the provisions for the feed and outflow of the dialysate. Single-pass systems are almost exclusively used today, i.e., systems in which the continuously fed dialysate passes only once through the dialyzer and is then rejected. The dialysate is continuously generated by mixing water with a concentrate in a proportional mixing system. The dialysate must be degassed and warmed to body temperature before it can be fed to the dialyzer. These steps require great technical expenditure resulting in a very high cost for the apparatus. The susceptability of such apparatus to malfunction must also be taken into account. There can be faulty connections, or for example, a deviation from the correct mixing ratio or over-heating of the dialysate. This can place the patient being treated in mortal danger. It is therefore indispensable to equip such apparatus with monitoring devices, which further increases the costs of acquisition, monitoring, and maintenance of the apparatus.
The control of the amount of water withdrawal through ultrafiltration during hemodialysis treatment presents a special problem. Conventional single-pass systems lack an exact balancing of the dialysate flowing from the dialyzer with the dialysate fed into it. The usual method for ultrafiltration control is by adjusting the pressure difference between the blood side and the dialysate side of the semipermeable membrane of the dialyzer. Given the ultrafiltration characteristic of the particular dialyzer used, i.e., the relation between the pressure difference and the amount of fluid per unit time passing through the membrane (ultrafiltration), the pressure difference, the so-called transmembrane pressure, necessary for the desired ultrafiltration rate can be set. This method, however, is subject to interference and inaccuracies, especially because the ultrafiltration characteristic is subject to considerable variation and can be altered by various inferring influences, such as deposits on the membrane. In critical cases, therefore, special monitoring of the patient's weight decrease with a bed scale is usual.
In addition to the single-pass apparatus there are also the so-called tank kidneys. German published application DE-AS No. 22 59 787 describes, for example, a much simplified hemodialysis apparatus that operates with a rigid volume container sealed off against the atmosphere. The container is completely filled with fresh dialysate before the start of treatment. During operation, dialysate is pumped from the container through the dialyzer and the used dialysate is conducted back into the container. Because of the constant volume of the total system, ultrafiltration can take place only when liquid is withdrawn from the system. This withdrawn liquid is replaced by ultrafiltration from the blood into the dialysate, so that the amount of liquid withdrawn equals the amount of ultrafiltrate. The ultrafiltration rate, accordingly, can be controlled relatively simply in this system.
The apparatus according to German published application DE-AS No. 22 59 787 has, however, the disadvantage when compared to the usual single-pass systems, in that used dialysate is mixed with fresh dialysate. The efficiency of the process is thus reduced, since the concentration difference across the semipermeable membrane determinative of dialysis rate is reduced. This is one of the reasons why such apparatus have not been able to hold up in actual practice.
German published application DE-AS No. 22 59 787 discloses an electrically operated heating and regulating device for heating and maintaining the temperature of dialysate at body temperature. Aside from the technical expenditure, an electrically operated heating device on a hemodialysis apparatus has the fundamental disadvantge of a possibly endangering patient by electric currents. Since the dialysate is electrically conductive, it is not to be excluded that currents from the electrical heating device will pass over the dialysate, through semipermeable membrane and into the blood circulation of the patient. Since the patient's heart is further sensitized by the dialysis process, even slight currents present the danger of ventricular fibrillation.
With tank kidneys according to German published application DE-AS No. 22 59 787 there are also considerable hygienic problems. In a single-pass apparatus, the dialysate delivered from the mixing system and heated to body temperature is immediately used. Apparatus which holds the entire supply of heated dialysate in a container and has a separate heating arrangement in the dialyzate circulation presents ideal multiplication conditions for microorganisms in the dialysate. Such tank systems are also no longer generally regarded as acceptable because of this reason.